1. Field of the Invention
The present invention relates to an electronic car bumper in which the bumper, by employing a high-frequency proximity switch, can stably and surely detect an object such as a human body approaching a car and give an indication or a warning.
2. Description of the Related Art
Bumper system such as supersonic system or photo-electric reflection system, which can detect an object approaching a car, are now on the market. As shown in FIG. 1, such systems are installed on or in a bumper 1 in such a manner that they emit supersonic waves or light rays 3, reflected waves of which can be sensed by a sensor 2. Each of these systems, equipped at the corner of the car, have a small range of sensitivity so that they can cover the rear or the entire front part of the car. It is not desirable for the range of sensitivity of the car to vary widely or to spoil the beauty of the car by drilling holes for installing the sensors. The systems have advantages in that a relatively long detecting distance can be obtained and sensor portions (right and left) can be found since they are provided at the corners of the car, while a sensitivity distance of 2-3 [m] is not required for a use of this type. It is more important to have a uniform range of the sensitivity.
Therefore, it has been proposed to apply a high-frequency proximity switch as described in Japanese Patent Laid-open (KOKAI) No.186773/1985. The principles of the proximity switch are as follows: One portion of a high-frequency resonance circuit provided with a capacitor having parallel plate electrodes. One electrode thereof is grounded and the other electrode serves as a sensor plate. When an object such as a human body approaches the latter electrode, the capacitance of tile capacitor is increased. This influences an oscillation condition and makes an output of the oscillation vary. Then, the output is detected and switched.
As described above, an obstacle can be detected by applying the high-frequency proximity switch, but a position of the obstacle cannot be detected. To detect the obstacle, two or three high-frequency proximity switches are needed for one bumper, which results in increased and complicated wirings depending on the number of the proximity switches. If the various circuit systems having different frequencies simultaneously oscillate at close range, the systems interfere with each other and a malfunction occurs, and adjustment of the systems is required by providing with a tank circuit. The capacitance of a bumper varies due to exposure to the direct sun and due to wetness of rainwater or snow. In this case, a range of a temperature drift of a detected output is larger than that of a detected signal, so that a malfunction may occur.
It has been proposed that a high-frequency proximity switch be provided in a car bumper which can detect a man or an obstacle or the like and gives a warning sound as described in Japanese Patent Publication (KOKOKU) No.26414/1990. As shown in FIG. 2, the principle of the car bumper is as follows: The electronic car bumper comprises an oscillation circuit 4, sensor plates 8 including a plurality of electrode plates 6 and 7, a resonance circuit 9 including a loading coil L.sub.2 and a divided capacitor C.sub.3. A resonance frequency f.sub.01 the resonance circuit 9 is made equal to a resonance frequency f.sub.0 of the oscillation circuit 4. If a human body or the like approaches the sensor plates 8, floating capacitance Cc is increased and the resonanace frequencies f.sub.0 and f.sub.01 are detuned so that an oscillation output is decreased and an approach by a human body or the like can be detected by way of a change of this signal output.
FIG. 3 shows the capacitance which is produced when a person 10 approaches a car 19 equipped with such an electronic car bumper. Capacitance C.sub.T between the person 10 and the electrode 6, capacitance C.sub.0T between the person 10 and the car body, and capacitance C.sub.E between the car body and a ground are added to the capacitance of the sensor plates 6 and 7 and an oscillator reacts so that an object or the like is detected. If the person 10 approaches the car 19 while the car 19 is parked, nothing will happen. However, if the person 10 approaches the car 19 while the car 19 is in motion, some malfunction may occur. FIG. 4 shows this relationship.
That is, an electric line of force (a high-frequency current) E radiated from the electrode 6 generates two electric fields. One is an electric field E.sub.0 flowing through the capacitance C.sub.0 occurring between the car body and the electrode 6. The other is the electric field E.sub.E flowing through capacitance C.sub.E1 between the ground and the electrode 6. The electric field E.sub.E flows through capacitance C.sub.E0 generated between the ground and the car body to the electrode 7. If the car 19 is operated in such a condition, the capacitance C.sub.E0 between the car body and the ground and the capacitance C.sub.E1 between the electrode 6 and the ground oscillate. This means that the distance (capacitance) between the electrodes 6 and 7 varies generating capacity modulation. Therefore, when the sensitivity of the car bumper is increased, a malfunction may occur.